There are currently an estimated 40,000 incidences of cardiac arrest every year in Canada, most of which take place outside of hospital settings. The odds of an out-of-hospital cardiac arrest currently stand at approximately 5%. In the U.S., there are about 164,600 such instances each year, or about 0.55 per 1000 population. It may be desirable to decrease the number of deaths resulting from these out-of-hospital incidences of cardiac arrest. Certain places, such as sports arenas, and certain individuals, such as the elderly, are at particular risk and in these places and for these people, a convenient solution may be the difference between survival and death.
Cardiopulmonary resuscitation (CPR) is a proven effective technique for medical and non-medical professionals to improve the chance of survival for patients experiencing cardiac failure. CPR forces blood through the circulatory system until professional medical help arrives, thereby maintaining oxygen distribution throughout the patient's body. However, the quality of CPR is often poor. Retention of proper CPR technique and protocol may be inadequate in most individuals and the anxiety of an emergency situation may confuse and hinder an individual in delivering proper treatment.
According to the Journal of the American Medical Association (2005), cardiopulmonary resuscitation (CPR) is often performed inconsistently and inefficiently, resulting in preventable deaths. Months after the completion of standard CPR training and testing, an individual's competency at performing effective chest compressions often deteriorates significantly. This finding was found to hold true for untrained performers as well as trained professionals such as paramedics, nurses, and even physicians.
The International Liaison Committee on Resuscitation in 2005 described an effective method of administering CPR and the parameters associated with an effective technique. Parameters include chest compression rate and chest compression depth. Chest compression rate is defined as the number of compressions delivered per minute. Chest compression depth is defined as how far the patient's sternum is displaced by each compression. An effective compression rate may be 100 chest compressions per minute at a compression depth of about 45 cm. According to a 2005 study of actual CPR administration at Ulleval University Hospital in Norway, on average, compression rates were less than 90 compressions per minute and compression depth was too shallow for 37% of compressions.
Therefore, a system to facilitate the proper delivery of CPR in an emergency may be useful. Furthermore, a system that can also be used in objectively training and testing an individual may be useful for the CPR training process and protocol retention.
Most existing CPR assist technologies use accelerometers for the determination of compression depth. One such device is disclosed in U.S. Pat. No. 7,074,199. However, any acceleration data from accelerometers used to measure the depth of chest compression during CPR is prone to cumulative errors and drift errors. Consequently, these sensors are not suitable for highly accurate or detailed data collection regarding CPR parameters and can only be relied on for approximate depth values. Furthermore, the use of an accelerometer in a CPR monitoring device without an external reference is prone to error if the patient or CPR administrator is mobile. For example, if the patient is being medically transported in an ambulance, helicopter or on a gurney, the accelerometer is unable to differentiate between the external movement of the patient and the compressions of the chest. In any type of non-stationary environment, an accelerometer based device may be unreliable and ineffective. The use of an accelerometer to calculate compression depth also relies on complicated and error-prone calculations to compensate for the angle and tilt of the compression device. If the accelerometer is not perfectly level on the chest of the patient and its movement is not perfectly vertical, errors may accumulate and must be accounted for by the angle of the two horizontal axes. Furthermore, the absence of any external reference point makes it difficult for the device to know its position in space at any given time. All measurements of distance are relative and an origin of movement is difficult to ascertain and maintain over the course of measurements. This may cause the initiation or starting point of the compressions to drift over time leading to errors in depth measurements. Certain commercial products currently use accelerometer technology, such as the AED Plus® D-Padz® from Zoll Medical, in which the accelerometer is embedded into the pads of the defibrillator. Due to the additional circuitry and sensory within them, these defibrillator pads are substantially more expensive and must be disposed of after each use. Therefore, relatively expensive sensory must be routinely discarded due to the design of the product.
U.S. Patent Application Publication No. 2007/0276300 to Kenneth F. Olson et al. discloses a device using ultrasound transmission to calculate compression depth. An acoustic signal is transmitted from a device on the chest of the patient to a receiver in another location. This device has several drawbacks. First, the ultrasound signal must have a clear line of sight from transmitter to receiver in order to operate. Any interference, objects, people or even the hand of the user in the way of the signal may result in signal loss or deterioration. The transmitter must be directed toward the receiver and the relative orientation between the transmitter and receiver is crucial. Second, ultrasound is relatively slow and a time-of-flight measurement of an ultrasound signal may suffer from significant lag and latency. Third, an ultrasound signal is highly dependent on ambient conditions such as air temperature. If air temperature fluctuates, so does the speed of sound, which may result in inaccuracies. Finally, if the plane of the chest compression is initially unknown, the calculation of compression depth may be significantly compromised. Time-of-flight ultrasonic distance interpolation cannot resolve the position of the receiver in six degrees of freedom and the determination of the downward translational movement if the patient, receiver or transmitter is not level may be difficult. Even if ultrasonic triangulation is employed, latency may be significant, resolution may be low and multiple transmitters and receivers in different locations may be required.
Existing CPR assist devices and systems are relatively ineffective at measuring chest recoil. Chest recoil is the extent to which the chest is released following a compression. For a chest compression to be completely effective, the chest must be fully released before beginning another compression. When a compression is released, elastic recoil will create a negative pressure that pulls blood into the chest. Incomplete decompression will reduce the amount of blood available to be circulated with the next compression. Accelerometer-based devices lack the ability to establish a reference point at the top of a compression that may be used to adequately measure recoil. As there is no external reference, the accelerometer signal may drift over time and the device may become ineffective at determining whether the chest has been fully released.
A recent study (Resuscitation. 2009 January; 80(1):79-82. Epub 2008 Oct. 25: ‘Compression feedback devices over estimate chest compression depth when performed on a bed’) has unearthed another inadequacy in current CPR assist devices. The study indicates that CPR assist devices tend to overestimate compression depth when the patient is on a mattress. The device tends to erroneously register the movement of the mattress as part of the chest compression.
Other CPR assist tools use mechanical force measurements as an indication of compression depth. These devices may be inaccurate due to their inability to compensate for varying chest compliances. They tend to rely on the user's subjective impression of the patient's body size to help calibrate the proper amount of force to be administered. Furthermore, a recent study (Resuscitation. 2008 July; 78(1):66-70. Epub 2008 Apr. 18: ‘Does use of the CPREzy involve more work than CPR without feedback?’) has shown that these devices tend to require more work than CPR without an assist tool due to the device's internal mechanism. The spring within the device may add an additional 20% workload to the CPR process leading to a faster onset of user fatigue.
Presently available CPR assist devices and system typically suffer from a major disadvantage. They tend to indirectly measure depth by first determining acceleration, velocity or force. Ultimately compression depth is a measure of position and the determination of acceleration requires doubly integrating the received signal to obtain useful data. Such integration introduces a significant source of error into the measurement. It may be desirable to provide a method of determining CPR compression depth by measuring position, rather than acceleration, velocity or force. By measuring position directly, errors related to integration of the signal or compliance of the patient's chest are not introduced. The position data may be used to directly calculate the depth of chest compressions.
It may be desirable to provide an easy-to-use and inexpensive system to accurately measure relevant CPR parameters such as compression depth and rate absent of the problems in the aforementioned technologies.